Conventionally, data communication systems have used narrow band modulation techniques, such as amplitude modulation, frequency modulation, and binary phase shift keying. With such systems, demodulation at the receiver can be achieved with a relatively small amount of circuitry. Such systems, however, suffer from several problems, including multipath fading and narrow band noise.
By contrast, in spread spectrum communication systems, a data spectrum is spread by a pseudo-noise code ("PN code") at a transmitter, while the PN code and the data are synchronized at a receiver so that the adverse effects of multipath fading and narrow band noise can be reduced. The characteristics of spread spectrum communication systems also have been used by the military to combat intentional jamming of radio and satellite communication links or to make it difficult to detect such transmitted signals. Accordingly, spread spectrum communication systems have attracted increased attention as a promising technique for radio frequency transmission of binary data.
The PN code typically is defined by a binary sequence and is often referred to as the "chip sequence". The binary symbols in the chip sequence are referred to as chips and it is assumed that the transmitter and intended receiver both have available the same chip sequence.
One of the two most common spread spectrum techniques called frequency hopping uses the chip sequence to shift over a wide bandwidth the carrier frequency of a conventional narrow band transmitter signal. The other common technique, called direct sequence spreading, directly multiplies a conventional narrow band signal by the chip sequence where the chip rate typically is much higher than the data rate. In both of these common spread spectrum techniques, a conventional narrow bandwidth communications signal can be viewed as a carrier which is frequency modulated or directly multiplied by the chip sequence. There are other types of spread spectrum systems including combinations of these two basis types in one system.
Spread spectrum signals may allow more than one transmission signal in the same frequency and time interval where each such signal uses a different chip sequence. This technique is called code division multiple access (CDMA). One application of Direct Sequence CDMA (DS-CDMA) is the Global Positioning System (GPS), which uses DS-CDMA to broadcast time and position data to receivers, which can use such signals to determine position and navigation information.
The subject of spread spectrum communications is given in a three book series by Marvin K. Simon, Jim K. Omura, Robert A. Scholtz, and Barry K. Levitt, Spread Spectrum Communications, Volume I, II, and III, Rockville, Md.: Computer Science Press, 1985. See also Robert A. Scholtz, The Origins of Spread-spectrum Communications, IEEE Transactions on Communications, COM 30, pp. 822-854, May 1982; Rober A. Scholtz, Notes on Spread-spectrum History, IEEE Transactions on Communications, COM-31, pp. 82-84, January 1983; and Robert Price, Further Notes and Anecdotes on Spread Spectrum Origins, IEEE Transactions on Communications, COM 31, pp. 85-97, January 1983.
Since the spectrum of the information signal in a spread spectrum system is spread by a PN code having a broader spectrum width, in order to correctly restore the information signal, it is necessary to synchronize the demodulation PN code generated at the receiving side with the modulation PN code generated at the transmitting side. Proper phase synchronization is typically achieved when the received spread spectrum signal is accurately timed in both its spreading PN code pattern position and its rate of chip generation. The phase synchronization process typically is accomplished in two stages: an initial synchronization process for finding a synchronous phase, and a process for tracking the detected phase. Known techniques for attaining the initial synchronization depend upon both analog and digital sliding correlators, matched filters and other devices.
In a conventional matched filter spread spectrum receiver, the receiver includes a radio frequency (RF) section for receiving the spread spectrum signal having a PN code modulated therein. The receiver converts the received spread spectrum signal into an intermediate-frequency (IF) signal. An in-phase converter and a quadrature-phase converter convert the IF signal into an in-phase (I-channel) spread signal and a quadrature-phase (Q-channel) spread signal. A PN code sync device de-spreads the received PN code modulated from the spread spectrum signal by synchronizing a reference PN code with the received PN code and maintaining the two codes in fine synchronism using, for example, a pair of correlators or a tracking loop based on a matched filter. A data demodulator demodulates the spread spectrum signal into the original baseband signal. The use of a matched filter has an advantage in that the transmitted coded signal can be acquired relatively quickly even with relatively large initial errors between the locally generated PN code and the received PN code.
In a DS-CDMA system such as a GPS receiver, a matched filter can be used to receive signals from multiple transmitters using a single set of receiver samples. However, each signal has a different PN code and a different amount of Doppler shift. If the receiver samples are Doppler corrected before being stored in the matched filter, either new signal samples or new Doppler correction samples have to be created and used for each desired signal to be received. This lowers the rate at which the matched filter can be used and also increases the power consumption of the receiver due to the filter loading process.
Accordingly, the inventor has determined that it would be useful to have a Doppler corrected spread spectrum receiver that avoids the limitations of the prior art, and in particular such a receiver that is low power and fast. The present invention provides such a system.